Radon-222 (Rn-222), a daughter product in the uranium-238 (U-238) decay series, is present in a wide range of concentrations in all natural water supplies. Public water supplies from surface sources have near zero Rn-222 concentrations, ground water sources have intermediate levels (up to several thousand Bq/m3), and private wells the highest levels (some exceeding 10 6 Bq/M 3). The latter two sources serve about 50% of the U.S. population. Despite these high levels, the consequences of ingesting radon in drinking water has attracted little attention compared to those of airborne radon which has received a great deal of publicity. This is perhaps due to the lack of experimental data needed for risk estimation as pointed out by Sullivan and Nelson in a 1982 memorandum to the USEPA. The majority of the data on radon laden drinking water concerns calculation of the absorbed dose to the whole body, lung, kidney, or stomach. Little or no data are available which directly quantifies the biological response of mammalian tissue from ingested radon. The central aim of the proposed work is to f ill this gap by ascertaining the radiotoxic effects of radon laden water on mouse testis. The mouse testis model, with spermatogonial cell killing and induction of sperm-shape abnormalities serving as the end points, is an excellent biological dosimeter for studying the effects of extremely low doses (as low as 1 mGy of alpha-radiation) from minute quantities of incorporated radionuclides. Hence, in view of its sensitivity and its established relevance to man, this model is uniquely suited for this study. Since Rn-222 decays to a number of unstable daughter products (Po-218, Pb-214, Bi-214, and Po-214) which are present in the water and may contribute to the testis dose, the kinetics of testicular uptake and clearance of both radon and its daughters will be studied following oral administration. These studies will be followed by radiotoxicity experiments for each end point, spermatogonial cell survival and induction of sperm-shape abnormalities. Using these data and the kinetics data, dose response curves will be obtained thereby relating the ingested activity and testicular absorbed dose to the biological response. These data, which quantify the biological response in an organ of genetic importance, will be of significant value in assessing the risk from ingesting radon laden water.